The present invention relates to differential gear assemblies having an improved arrangement for mounting gear pairs.
A differential gear assembly of a type contemplated for use in the present invention is disclosed in U.S. Pat. No. 2,859,641 to V. E. Gleasman (co-inventor of present invention). The patent discloses a differential mechanism which may be mounted, for example, in the drive train of a motor vehicle between drive axles. The differential mechanism includes side gears in the form of worm gears coupled to inner ends of each drive axle together with combination gears having central worm wheel portions in mesh with the side gears and spur gear portions at either end in mesh with each other for transferring and dividing torque between the drive axles. The combination gears are provided in groups of one or more pairs, and each combination gear of a pair rotates on an axis of rotation that is substantially perpendicular to a common axis of rotation about which the side gears rotate. Ends of each combination gear terminate in stub shafts which are supported in bearings formed in removable posts or pedestals. A housing which is rotatable about the drive axles is provided with a flange at one end and a cap at the other end for securing the removable pedestals in the housing. A ring gear is attached at the flanged end of the housing for connecting the assembly to a power source.
This type of differential assembly may be referred to as a "cross axis compound planetary differential." Although the just-described differential works quite well, it is difficult to assemble and costly to manufacture. In order to install combination gears within the differential assembly, the housing must be disassembled to permit adjustment of the removable pedestals along the stub shafts. All of the combination gears must be simultaneously mounted between their associated pedestals before they can be installed in the housing. The removal or replacement of a single combination gear requires the assembly to be substantially disassembled, thereby disturbing previously established mounting positions of the other combination gears. In addition, extensive machining operations are required to permit accurate mounting of the removable pedestals within the assembly. These operations add considerable cost to the manufacture of the above-described device.
U.S. Pat. No. 2,631,475 to the same V. E. Gleasman discloses an earlier version of a cross axis compound planetary differential design which features a cartridge unit for simultaneously installing two pairs of combination gears within a differential housing. Both pairs of combination gears are mounted between end plates of a single cartridge unit. The combination gears in this prior design include a central worm gear portion and the side gears take the form of worm wheels. Each combination gear also includes a through bore for receiving a mounting shaft which is supported in bearings formed in end plates of the cartridge unit. A housing is provided with a removable end cap to permit insertion of the cartridge unit within the housing.
Although this design permits relatively easy assembly, the design was abandoned in favor of the first-discussed design of U.S. Pat. No. 2,859,641 on account of strength considerations. The use of cartridge end plates straddling the side gears required removal of excessive amounts of housing stock leaving little material with which to attach a housing end cap. It may also be noted that only two pairs of combination gears could be accommodated in the cartridge design. This consideration also limited the amount of torque which could be conveyed by the design.
A more recent cross axis compound planetary differential in current use and featuring a combination gear mounting design is disclosed in U.S. Pat. No. 3,884,096, also to V. E. Gleasman. The combination gears include worm wheel and spur gear portions and the side gears take the form of worm gears as first described above for U.S. Pat. No. 2,859,641. Similar to next-described U.S. Pat. No. 2,631,475, the combination gears also include through bores for receiving mounting shafts. A housing is provided with fixed pedestals which are spaced apart from each other and have parallel opposing faces for locating combination gear pairs within the housing. The mounting shafts extend into bearings formed through the parallel faces of adjacent pedestals and are retained in the housing by pins fastened at the exterior of the housing.
The design of U.S. Pat. No. 3,884,096 provides for the individual mounting of combination gears between parallel faces of adjacent pedestals which are formed integrally with one end of a differential housing. Although such a design possesses adequate strength, the mounting of combination gears individually is quite time consuming. In addition, carefully aligned and costly bores are required to be formed through the housing to permit the mounting of combination gears individually within the housing. Special pin arrangements are also required to retain the mounting shafts within the assembly. The failure of one of these pin arrangements could lead to an undesirable situation in which a mounting shaft released from the housing becomes jammed between other drive line components.